1. Field of the Invention
The present invention relates to an ignition timing control system and method for an internal combustion engine, which feedback-control ignition timing of the engine, and an engine control unit.
2. Description of the Related Art
Conventionally, as an ignition timing control system for an internal combustion engine, the present assignee has already proposed one disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2008-57438. This internal combustion engine is a gasoline engine which includes a plurality of cylinders, and the cylinders each have an in-cylinder pressure sensor mounted therein, for detecting in-cylinder pressure generated therein. In this ignition timing control system, a crank angle position at which the in-cylinder pressure becomes largest is calculated as a largest in-cylinder pressure angle, based on the detected in-cylinder pressure and the crank angle position, and the ignition timing is controlled by feedback control using a sliding mode control algorithm such that the largest in-cylinder pressure angle converges to a predetermined target angle.
FIG. 19 shows changes in in-cylinder pressure occurring from a the compression stroke to an expansion stroke. It should be noted that a curve M in FIG. 19 shows motoring pressure generated when combustion is not performed in a cylinder. When a combustion state in the cylinder is stable, a peak at which the in-cylinder pressure becomes largest clearly appears (see a curve A), whereby it is possible to accurately calculate the above-described largest in-cylinder pressure angle. However, if an EGR amount is large or load on the engine is small, for example, there occurs reduced combustion speed or increased combustion fluctuation, which prevents the peak of the in-cylinder pressure from appearing clearly (see a curve B), and hence there is a fear that it is impossible to accurately calculate the largest in-cylinder pressure angle. Further, if combustion pressure generated by combustion is smaller than a largest value of the motoring pressure (see a curve C) due to a large retarded amount of the ignition timing, the crank angle position of the largest value of the motoring pressure is erroneously calculated as the largest in-cylinder pressure angle. In these cases where the proper largest in-cylinder pressure angle cannot be obtained, it is impossible to carry out most appropriate ignition timing control. That is, in this ignition timing control system, the control range in which most appropriate ignition timing control can be carried out is narrow, which causes degraded fuel economy or increased combustion fluctuation.